The present invention relates to shutters for windows, doors or the like in general, and more particularly to improvements in shutters of the type wherein a contractible, expansible, liftable and lowerable curtain of light-intercepting elements in the form of lamellae or slats is movable up and down between two upright frame members and the upper end portions of the frame members are adjacent to a rotary main shaft which can be driven in two directions to move the slats up or down.
German Offenlegungsschrift No. 27 44 930 discloses a shutter wherein the curtain of slats moves up or down with a horizontal bottom rail which is disposed below the lowermost slat and cooperates with means for preventing unauthorized lifting of slats (e.g., by a burglar) when the curtain is fully extended. The end portions of the slats and of the bottom rail extend into and are guided by two upright guide rails which flank the curtain of slats and confine flexible lifting elements in the form of bands, tapes, cords, ropes or the like. The lower end portions of the lifting elements are attached to the respective end portions of the bottom rail and, when such flexible elements are slack (such as in the fully extended condition of the curtain), they can be moved aside by spring-biased levers which are installed in the guide rails and overlie the respective end portions of the bottom rail to prevent unauthorized raising of the curtain. However, the curtain of slats can be readily lifted by a person who has access to the mechanism or motor for the lifting of flexible elements, namely, for winding of such flexible elements onto reels which are provided on the customary main shaft extending horizontally between the guide rails at a level above the curtain of slats.
The just described safety feature is relatively simple and inexpensive. However, it also exhibits a number of rather serious drawbacks such as that the slats cannot be blocked or locked against unauthorized lifting in partly extended or expanded condition of the curtain. Moreover, the flexible lifting elements are subjected to extensive wear because they are engaged and flexed by the respective springbiased levers whenever the bottom rail is caused or allowed to assume its lower end position. The levers move into direct frictional engagement with and invariably ride along the same portions of the flexible elements so that the useful life of such elements is relatively short. Still further, snow, rain, dirt and/or other foreign matter which penetrates into the guide rails is likely to interfere with pivotal movements of levers under the action of their springs so that the bottom rail cannot be engaged at all, even in the fully extended condition of the slat curtain. Foreign matter is also likely to interfere with or to prevent downward movement of the bottom rail to its lower end position; this also prevents the levers from engaging the bottom rail so as to hold the slats against unauthorized lifting. Such engagement is impossible or unlikely, even if the bottom rail is arrested at a short or very short distance (e.g., one or more centimeters) from its lower end position. Finally, the overall weight of the conventional shutter is unduly increased by the bottom rail which must be sufficiently heavy to push the levers aside by gravity during movement to its lower end position, i.e., to overcome the resistance of springs which pivot the levers into engagement with the bottom rail as soon as the latter reaches its lower end position. The springs must be relatively strong since they should stand reasonably long periods of use and also because they must cause the levers to flex the lifting and/or tilting elements during pivotal movement into arresting or locking engagement with the respective end portions of the bottom rail.